Considering the actual structure of a human joint in vivo, it is preferable that ciliary substance is arrayed on a sliding surface of an artificial joint member, so that body fluid serving as a lubricating liquid is retainable in the cilia-like substance to provide the sliding surface with a satisfactory lubricating state. It is, however, difficult to artificially create such a state, and it is furthermore difficult to retain such a satisfactory state for a long term. None of the artificial joints has accomplished such a satisfactory state.
Observing the current status of the medical field, combined use of a soft material such as a polymer and a hard material such as a metal is popular for an artificial joint member such as a femoral ball and an acetabular cup. For example, artificial joints each comprised of a metallic femoral ball using stainless steel, cobalt chromium alloy, titanium alloy, or the like and a polymeric acetabular cup using ultra high molecular weight polyethylene (hereinafter, simply called as “UHMW polyethylene”) or the like have been implanted in several ten thousand surgeries conducted in a year in Japan. Replacement with such artificial joints is beneficial in relieving patients from pain and releasing them from bedridden state, thus improving their quality of life to a level similar to that of healthy people.
In the aforementioned artificial joint, friction between the metallic member and the polymeric member is generated on the sliding part of the artificial joint frequently owing to physical actions of a person implanted with the artificial joint in his or her daily life. Particularly, wear becomes serious on the side of the polymeric member of the artificial joint. Wear debris of polymer such as UHMW polyethylene is likely to induce osteolysis (bone dissolution), which may cause so-called “loosening” of weakening cohesive strength between the bone and the artificial joint member, and obstruct the function of the artificial joint. The wear debris of UHMW polyethylene generally amounts to 0.1 to 0.2 mm in thickness per year. Generation of wear debris may not be harmful for a certain time after the surgery. However, the loosening becomes intolerable in 5 years or so after the surgery, and replacement with a new artificial joint member is required.
In recent years, proposed is use of so-called (crosslink) polyethylene as means for suppressing wear of the polymer member of the artificial joint. (Crosslinked) polyethylene is a substance in which each molecule of UHMV polyethylene is rendered to a cross-linked state by irradiating a gamma ray or an electron beam onto the UHMW polyethylene. There has been reported that wear debris of such (crosslinked) polyethylene is reduced as low as ⅕ to 1/10 in quantity compared with that of ordinary UHMW polyethylene which has not been subjected to irradiation (Biomaterials, Vol. 20 (1999), pp. 1659-1688). However, practice of clinical application of (crosslinked) polyethylene is insufficient, and usability thereof has not been verified.
There has also been proposed combined use of a femoral ball and an acetabular cup both of which are made of a hard material, in place of using a polymer such as the aforementioned UHMW polyethylene which wears easily. For instance, combined use of a femoral ball of cobalt chromium and an acetabular cup of cobalt chromium alloy (Clinicalorthopaedics and Related Research, No. 333 (1996) pp. 96-107), and combined use of a femoral ball of alumina and an acetabular cup of alumina (The Journal of Arthroplasty, Vol. 14, No. 7 (1999) pp. 781-787) are applied to an artificial hip joint clinically. In spite of such a clinical effort, long-term use of the cobalt-chromium femoral ball and the cobalt-chromium-alloy acetabular cup is considered hazardous in that wear debris generated by friction between the cobalt chromium alloys has toxicity to cells in itself.
Further, since the artificial joint comprised of the alumina femoral ball and the alumina acetabular cup may be broken during a surgery operation or in use after the surgery because the alumina is brittle, further improvement is desired to accomplish a successful practical use.
In addition to the above, the aforementioned hard materials are rigid with less elasticity, and do not exhibit cushionability like UHMW polyethylene. Such rigid materials are not desirable because they do not exhibit shock absorbing action to an external force, and a load is directly exerted to the bone.
In view of the aforementioned problems residing in the prior art, it is an object of the present invention to provide an artificial joint member made of a polymeric material which is capable of suppressing generation of wear debris under repeated use in daily life by suppressing friction at a sliding part thereof, and assuring long-term use by providing the artificial joint member with shock absorbability.